Materials in nanoscale may possess new physical and/or chemical properties. For the development of nanotechnology, the most fundamental and important issue is the simplicity and controllability of the preparation method of nanoscaled materials.
There are a large number of reports on the preparation of nanoscaled materials, but there are still some nanoscaled materials that cannot be prepared by the current methods, such as nanoscaled metal complex hydrides, which are not thermodynamically stable enough and extremely chemically active (can react with H2O and O2). No method exists for the synthesis of nanoscaled complex hydrides with less than 50 nm in size up to now.
The emission of greenhouse gasses and the depletion of non-renewable energy resources are two critical problems for human beings. Hydrogen energy is one of the promising solutions to the problems. Hydrogen energy is based on the reaction between H2 and O2 to produce energy and H2O, which is absolutely clean. At present, the efficient and safe hydrogen storage techniques are the key barrier that prevents hydrogen energy from mobile applications (such as fuel cell vehicles and portable computers and cell phones).
In the past decades, researchers have developed four hydrogen storage techniques, i.e., liquid hydrogen storage systems, compressed hydrogen gas storage systems, cryo-adsorption hydrogen storage systems, and hydrides solid hydrogen storage systems. Among them, liquid hydrogen storage systems and compressed hydrogen gas storage systems are mainly utilized for large scale or stationary purpose due to their heavy and expensive containers.
Cryo-adsorption hydrogen storage systems have relatively high gravimetric and volumetric hydrogen storage densities (e.g. activated carbon can store 4.5 wt % of hydrogen at liquid N2 temperature, see Ahluwalia R K, Peng J K. Automotive hydrogen storage system using cryo-adsorption on activated carbon. Int J Hydrogen Energy 2009; 34:5476-87), but the strict requirement of cryogenic conditions is difficult to obtain in daily life.
Hydrides that used as solid hydrogen storage materials can be divided into two categories: metal hydrides and metal complex hydrides. Metal hydrides, such as LaNi5H6 and MgH2, have been investigated in depth in the last few years. At room temperature, 1 mole of LaNi5 can absorb 6 moles of H atoms under higher hydrogen pressure, and then desorb/release them under lower hydrogen pressure. The disadvantage is the low hydrogen storage capacity (less than 1.5 wt %), which cannot meet the requirement of practical applications. MgH2 can store more than 7 wt % of hydrogen, but suffers from higher operating temperature (above 300° C. for desorption) and slow hydrogen charge/discharge kinetics, which also cannot meet the requirement of practical applications.
Metal complex hydrides, such as alanates, amides and borohydrides, have attracted a lot of attention recently due to their high gravimetric and volumetric capacities and relatively moderate hydrogen absorption/desorption thermodynamics/kinetics. For example, Ti-doped NaAlH4 can reversibly store 4.5 wt % of hydrogen at 130° C. (see Bogdanović B, Schwickardi M. Ti-doped alkali metal aluminum hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997; 253:1-9); K-modified Mg(NH2)2-2LiH composite can provide a reversible hydrogen capacity of 5.2 wt % at 130° C. (see Wang J, Liu T, Wu G T, Li W, Liu Y F, Araujo C M, Scheicher R H, Blomqvist A, Ahuja R, Xiong Z T, Yang P, Gao M X, Pan H G, Chen P. Potassium-modified Mg(NH2)2/2LiH system for hydrogen storage. Angew Chem Int Edit 2009; 48:5828-32); 2LiBH4—MgH2 composite can store more than 11 wt % of hydrogen reversibly at around 400° C. (see Vajo J J, Skeith S L, Mertens F. Reversible storage of hydrogen in destabilized LiBH4. J Phys Chem B 2005; 109:3719-22). Unfortunately, none of the materials can meet the technological requirements for fuel cells, which were set by the US Department of Energy (DOE).
Recent investigations reveal that reducing the particles to nanosize can significantly improve the hydrogen storage properties of hydrides. However, owning to the relatively low thermodynamic stability and extremely high chemical activity of the hydrides (easily reacting with H2O and O2), almost all the current nanofabrication methods is not feasible for hydrides, especially for the nanoscaled hydrides with unique morphologies.
Up to now, two indirect methods for the preparation of nanoscaled hydrides have been reported. One method is to confine metal hydrides in nano-scaffolds. Researchers found that NaAlH4 and LiBH4 could be impregnated into nanostructured carbon through solution submerge or high pressure melting permeatation. Encouragingly, the hydrogen absorption/desorption temperatures of the nano-confined materials were dramatically decreased in comparison with the raw materials (see Balde C P, Hereijgers B P C, Bitter J H, de Jong K P. Facilitated hydrogen storage in NaAlH4 supported on carbon nanofibers. Angew Chem Int Edit 2006; 45:3501-3 and Gross A F, Vajo J J, Van Atta S L, Olson G L. Enhanced hydrogen storage kinetics of LiBH4 in nanoporous carbon scaffolds. J Phys Chem C 2008; 112:5651-7). Carbon nanofiber supported NaAlH4 begins liberating hydrogen at as low as 50° C. (see Balde C P, Hereijgers B P C, Bitter J H, de Jong K P. Facilitated hydrogen storage in NaAlH4 supported on carbon nanofibers. Angew Chem Int Edit 2006; 45:3501-3). However, the loading efficiency of the original metal hydrides is quite low, and the huge dead weight of the scaffold lowered the overall hydrogen storage capacity to an inferior value.
The other method is to synthesize nanostructured metal first, and then hydrogenate to form nanostructured hydride. MgH2 nanowires synthesized with this method exhibit a uniform diameter of about 50 nm and could absorb/desorb 7.6 wt % of hydrogen at 300° C. in 30 min (see Li W Y, Li C S, Ma H, Chen J. Magnesium nanowires: Enhanced kinetics for hydrogen absorption and desorption. J Am Chem Soc 2007; 129:6710-1). However, the particle size of the samples significantly increases after the hydrogenation. Such method is only used for single-metal-element hydrides, but cannot be used for complex metal hydrides with more than two non-hydrogen elements.
A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands, extending in an array. It can also be described as a polymer whose repeated units are coordination complexes. The structure of a coordination polymer can be determined to be one-, two- or three-dimensional, depending on the number of directions in space to which the array extends. One type of special one dimensional coordination polymer is formed by metal coordination hydride and organic ligand, and the organic ligand can be removed under the condition that the metal hydride decomposes.
For example, Mg(AlH4)2.Et2O coordination polymer (see Fichtner M, Fuhr O. Synthesis and structures of magnesium alanate and two solvent adducts. J. Alloys Compd 2002:345:286-96), LiBH4.X coordination polymers (X is Et2O, MTBE (methyl tert-butyl ether) or THF (Tetrahydrofuran), see Giese H H, Noth H, Schwenk H, Thomas S. Metal tetrahydridoborates and tetrahydridometallates. 22—Structural chemistry of lithium tetrahydroborate ether solvates. Eur J Inorg Chem 1998:941-9 and Ruiz J C G, Noeth H, Warchhold M. Coordination compounds of alkali metal tetrahydroborates with ethers and amines. Eur J Inorg Chem 2008:251-66), Ln(BH4)2.2THF coordination polymers (Ln is Eu and Yb, see Marks S, Heck J G, Habicht M H, Oña-Burgos P, Feldmann C, Roesky P W. [Ln(BH4)2(THF)2] (Ln is Eu, Yb)—A Highly Luminescent Material. Synthesis, Properties, Reactivity, and NMR Studies. J. Am. Chem. Soc. 2012; 134:16983-6), and so on. These materials have a same structural feature, i.e., the molecular structure is one-dimensional chain-shaped, in which the complex hydrides act as a framework with organic ligands shielding their edge.